Note: After the quest updates with the message "I have arrived at Lake Arrius Caverns. The secret Mythic Dawn shrine must be somewhere inside" if one exits the cave at any time and then re-enters, the player won't be able to get back out. Be sure to obtain the Mysterium Xarxes before exiting the cave or else the main quest will be incompletable!

Upon entering the cave, the Hero will be greeted by a doorman who tells them to speak to Harrow. Harrow will ask them to give him all of their items and gold. Harrow will lead them into a dark sacrificial chamber where after a short speech from Mankar Camoran they will be invited to join the Mythic Dawn.

They will be prompted to kill the prisoner Jeelius and, if this is done, they will become an acolyte of the Mythic Dawn. Some of the cultists that were attending the ceremony will leave, making it reasonably easier to escape.

If deciding to release Jeelius, the cultists will attack him and the Hero. If Jeelius makes it out of the cave alive, he will reward them later with +1 to Alchemy, Restoration and Mysticism skills when visiting him in the Temple of the One in the Imperial City. Be warned though, as it can be difficult to keep up with him and protect him since he runs very fast.

Follow the marker in through the cave to the large cavern and kill everyone that attacks. Watch Mankar Camoran do his speech and then disappear before attacking any Mythic Dawn member, as everything picked up in the cave will be considered "stolen". Dispose of all the Mythic Dawn members and head up on the altar platform where the book is on a pedestal.

Regardless of which method chosen, Mankar Camoran will escape to his Paradise. Picking up the book will open a sealed gate to the north and the escape route. Jeelius has to be saved before picking up the book because otherwise he'll be killed by the collapsing statue of Mehrunes Dagon. Once the Hero has taken the book, escape from the caverns, and return to Martin in Cloud Ruler Temple.

I have followed the clues hidden in the four volumes written by Mankar Camoran to a gravestone in Green Emperor Way. At noon, a map of Cyrodiil appeared on the gravestone, with the location of the Mythic Dawn's shrine marked on it. I need to find my way into the shrine and locate the Amulet of Kings.

Update: Arriving at Lake Arrius Caverns, the location of the Mythic Dawn shrine:

I have arrived at Lake Arrius Caverns. The secret Mythic Dawn shrine must be somewhere inside.

Update: Getting past the first Mythic Dawn guard:

I have passed the first hurdle. The Mythic Dawn guard has let me into the secret shrine. Now to locate the Amulet of Kings.

Update: If you choose to give you armor and weapons to Harrow, the Warden of the Shrine, to become an initiate into the Mythic Dawn cult:

Harrow, the Warden of the Shrine, accepted me as an initiate into the Mythic Dawn cult. I was required to give him all my possessions and dress in an initiate's robe. He said that the Master, Mankar Camoran himself, is here at the shrine, and might perform my initiation himself. I should follow Harrow and see if he leads me to Mankar Camoran and the Amulet of Kings.

Update: After upsetting the Mythic Dawn cult and attacking everyone:

Mankar Camoran escaped to his realm of Paradise with the Amulet of Kings, and closed his magical portal behind him. But the Mysterium Xarxes itself is here in the shrine. I must get the Mysterium Xarxes. It is my only hope of finding a way to recover the Amulet now.

Update: After taking the Mysterium Xarxes:

I have the Mysterium Xarxes, the Mythic Dawn's book of secrets. Now I must escape this place and return to Cloud Ruler Temple.

Update: After escaping Lake Arrius Caverns:

I have escaped with the Mysterium Xarxes. Now to deliver it to Martin at Cloud Ruler Temple.

The chances that a random Oblivion Gate will appear reach the maximum level at any point during the game: the number of possible random gates increases to 50 (from 25), and the chance of a gate opening when the Hero enters its cell increases to 50% (from 25%).

Seven fixed gates open, one outside of each city (except Kvatch and the Imperial City). These gates are part of the Allies for Bruma quest. When asked about their city, NPCs will mention the Oblivion Gate outside their walls.

Followers who accompany the Hero into the caverns will attack the Mythic Dawn members, preventing the Hero from blowing their cover up until the point when they must take the Mysterium Xarxes.

Before heading in and getting all items taken away, drop any armor, weapons or potions desired on the ground. Then accept everything to be taken away, run back, and grab the gear dropped.

During this quest, after receiving the Mythic Dawn robes and before attacking the cultists, the Hero may say Mythic Dawn lines, such as "Die unbeliever" and "For Lord Dagon," while power attacking. Those lines will not be uttered anymore after attacking the cultists.

The Blade of Woe, obtained in the Dark Brotherhood questline, will not be taken by Harrow. Players experienced in the use of daggers may want to carry it on the temple.

Another option is to sneak past Harrow without talking to him. The cult members inside will not see them as an enemy even if not wearing the mythic garb. To do this, walk just around the corner until one sees Harrow, then walk backwards out of the hallway. When Harrow gets to the end, he will see the gatekeeper and turn around. As soon as he turns around, just run past him and through the door.

In the treasure room, down the ramp there is a random ring drop in one of the crates. The Storeroom key found on one of the members can be used to enter. Potentially, it is possible to get Ring of Perfection, which grants a +10 bonus to all stats. Also it may be possible to receive a Base Amulet of Absorption (Spell absorption 14pts).

If the Hero gives all of their items to Harrow and then decides to kill the Mythic Dawn in the shrine room, their items will all be on Harrow's corpse after he is killed.

After escaping from the shrine, Mythic Dawn agents will now attack on sight (due to a scripted faction-wide disposition decrease when the Hero picks up the Mysterium Xarxes). Some agents are posing as townsfolk. The only warning will be a scream, "For Lord Dagon!", before they summon their Daedric armor and attack the Hero in the street. However, it is usually easy enough to find a guard to cut them down, and the townsfolk will help as well.

If the Hero has killed an NPC and is not yet a member of the Dark Brotherhood, sleeping in any bed in the Shrine will prompt a visit from Lucien Lachance. His speech may be interrupted by members of the Mythic Dawn attacking him. A way to prevent this is to sleep somewhere where no Cultists near enough to detect him or by not sleeping at the Shrine at all.

The front cover of Oblivion's game box is also the cover of the Mysterium Xarxes.